This invention relates to a triggered ignition system and particularly to a system employing switching means for selectively supplying energy to the several ignition means with respect to a selected desired firing point.
Electronic ignition systems have recently been developed to provide improved ignition in internal combustion engines and the like. A highly satisfactory electronic system employs a capacitor which is charged to a relatively high voltage and then rapidly discharged thru a step-up ignition transformer to provide the firing energy to a selected spark plug. Such capacitor discharge ignition systems may employ a battery power supply in combination with a dc to dc converter for charging of the capacitor to the firing level or alternatively may employ an alternator coupled to and driven by the engine to produce an alternating output which is rectified and applied to charge the capacitor.
Capacitor discharge ignition systems and the like have also been developed with individual outputs for the several cylinders in order to eliminate the requirement for distributors and the like. Further, such systems may be advantageously constructed with special trigger signal generating circuits to eliminate the necessity for breaker points.
A very satisfactory capacitor discharge ignition system is shown in applicant's recently issued U.S. Pat. No. 3,805,759 entitled IGNITION SYSTEM WITH ADVANCE STABILIZING MEANS. As more fully disclosed in such application particularly as applied to an outboard motor or the like, an alternator is coupled to the flywheel of an internal combustion engine and connected via a rectifier circuit to charge a main firing capacitor. A separate signal generator is also coupled to the flywheel and establishes properly timed individual triggering signals. The output of the main firing capacitor is connected to a discharge network including a controlled rectifier, the gate of which is connected to the output of an appropriate trigger circuit to provide for the discharge of the capacitor for firing of the appropriate spark plug of the engine. A bias network is incorporated into the trigger circuit to prevent uncontrolled or erratic advanced firing which can result in a condition of engine speed instability and possible engine damage.
As described in U.S. Pat. No. 3,805,759 in connection with a two-cylinder outboard motor, the bias network creates a variable threshold voltage approximately matched to the variable trigger signal strength. Thus, at low speeds when the trigger signal is low, a low bias signal is introduced and, consequently, has little or no effect on the ignition timing. As the rotational engine speed increases which generates an increasingly strong trigger signal, the self-generating bias circuit introduces a corresponding larger opposing bias which must be overcome by the trigger signal. Applicant has found that the opposing bias network effectively neutralizes any change in the ignition angle with engine speed such as heretofore encountered. Further, with the bias network there is essentially no sudden large change of the ignition angle with a change in the angular position of the spark advance mechanism or with increasing engine speed; at most the change may be made to appear only as a relatively insignificant one-half degree jump in a practical outboard motor in the speed range of from 4,000 to 6,000 revolutions per minute. As a result, a desirable and consistent correlation between the ignition angle and the angular setting of the spark advance mechanism is established and is maintained stable for all speeds.
Such a self-generating bias network includes a parallel capacitor and resistor connected in series with the output of the trigger pulse generator and the triggering circuit means.
A multiple cylinder internal combustion engine ignition system employing a plurality of similar cascaded trigger and firing circuits for the several cylinders is also shown in applicant's co-pending application entitled "IGNITION SYSTEM FOR MULTIPLE CYLINDER INTERNAL COMBUSTION ENGINES HAVING AUTOMATIC SPARK ADVANCE", filed on May 10, 1973 with Ser. No. 359,137 now U.S. Pat. No. 387439. Diode and switching means connect the opposite polarity triggering pulses to the different firing circuits for a related pair of spark plugs to provide for an automatic spark advance at a selected speed.
A rotating magnet generator similar to that shown in U.S. Pat. No. 3,715,650 issued to James R. Draxler for a "PULSE GENERATOR FOR IGNITION SYSTEMS", is employed to generate relatively positive and negative pulse signals at oppositely located magnetic discontinuities. With any given single coil, it is merely necessary to rearrange the magnets to locate the discontinuities at appropriate points, to automatically generate a retarded of first polarity triggering pulse for a first spark plug in a first cylinder, and an advanced or second polarity triggering pulse for a second spark plug in a second cylinder, which pulses are conducted into the circuit by suitable switching and steering means connected to the opposite ends of each trigger coil or winding in accordance with the teaching of the above referenced application Ser. No. 359,137.
A self-biasing network is employed therein not only to stabilize the triggering but also to define a tachometer type signal directly related to the operating speed of the engine. The tachometer signal is applied to an electronic switching circuit to activate the second polarity signal circuitry to establish the automatic ignition angle advance.
Applicant has found that, although the biasing system of U.S. Pat. No. 3,805,759 provides highly significant improved results, and has further proved its usefulness by providing a tachometer type signal to initiate an automatic spark advance in the referenced co-pending application, the voltage which was developed across the bias capacitor was severely limited by the gate-to-cathode reverse voltage limitation of most controlled rectifier devices.
Thus, when the trigger generator is contructed to supply adequate trigger signals to assure easy engine starting at the low cranking speeds, the result is a relatively high peak extrapolated trigger voltage, generally in the order of 100 volts, at the maximum engine speeds. Extrapolatin is necessary to reveal the true nature of the high speed trigger signal inasmuch as a heavy added resistive load is applied to the trigger generator whenever the trigger signal exceeds the triggering threshold, which greatly reduces the observed peak voltage and naturally distorts the inherent trigger signal waveshape. The extrapolated high speed trigger signal can be readily observed by mechanically rotating the engine flywheel with the heavy added resistive load disconnected.
Applicant has found that the steepest portion of the leading edge of this extrapolated trigger pulse lies between one-third and two-thirds of the full peak extrapolated voltage. Triggering in the steepest portion of the leading edge of the extrapolated trigger pulses has been determined to provide the most precise ignition timing and the most precise spark angle relationships between cylinders. However, the back-bias stabilizing voltage must then be of the order of at least 35 volts.
Unfortunately, the reverse voltage blocking characteristic of the typical gate-to-cathode junction of a controlled rectifier is of a relatively low voltage level, generally of the order of 12 to 15 volts. Consequently, during the period of time that the triggering voltage is at or near zero, the gate-to-cathode junction of the controlled rectifier is subjected to the full voltage of the bias capacitor. Whenever the voltage of the bias capacitor tends to rise above the 12 to 15 volt range, the junction will conduct and permit a reverse current tending to drain the charge from the bias capacitor with a corresponding reduction in the back bias stabilizing voltage.